Modern semiconductor memory devices often use error checking and error correction bits to provide a reliable storage means for processors or other components. Generally, error-correcting code memory (ECC memory) is a type of computer data storage that may detect and/or correct the most common kinds of internal data corruption. ECC memory is used in most computers where data corruption cannot be tolerated under any circumstances, such as for scientific or financial computing.
Ideally, ECC memory creates a memory system in which the data that is read from each word or memory location is always the same as the data that had been written to it, even if a single bit (or more in some cases) that was actually stored has been flipped or changed to the wrong state (e.g., a “1” to a “0”, etc.). Traditionally a method of providing that memory protection is to use a Hamming code that is calculated based on the data portion of each memory word, typically 32 or 64 bits wide. Often, the Hamming code is chosen such that it can correct single bit errors in the memory word, and detect up to two total memory bits in error.
Some non-ECC memory with parity support allows errors to be detected, but not corrected; otherwise errors are not detected. In such a system, one or more extra bits of data are added to a memory. These extra bits indicate whether or not the actual or subject data includes an even or odd number of “1”s. Generally, with such a system the flipping of a single-bit within the actual data may be detected but not corrected.
Often the ECC code word or parity bit(s) are stored and fetched in parallel with the data word and the check is generated (for writes) and/or verified (for reads) as the memory access takes place. Generally, an immediate or substantially immediate correction or detection of errors is possible.